Cutting tools such as face mills, end mills, and in particular, throw-away cutting tools are widely used. Throw-away cutting tools, which are used by mounting cutting inserts on holders, are cost efficient.
For example, Patent Document 1 discloses a cutting insert comprising a main cutting edge (first cutting edge) at one corner and a secondary cutting edge (second cutting edge) at another corner. The cutting insert comprises a flank corresponding to the first cutting edge and a flank corresponding to the second cutting edge, the flanks being different from each other.
To be specific, in the cutting insert described in Patent Document 1, the flank corresponding to the first cutting edge is divided into a plurality of regions. Among the regions, a flank region A, which is connected to the first cutting edge, has the smallest clearance angle. A transition region B, which is connected to a lower side of the flank region A, has the largest clearance angle. A flank region C, which is connected to a lower side of the transition region B, has a clearance angle smaller than that of the transition region B. Moreover, the flank corresponding to the second cutting edge comprises basically a single surface having a clearance angle equal to that of the flank region C.
The flank region A, which is connected to the first cutting edge, comprises a segment I and a segment II. The segment I has a constant dimension in the thickness direction and a constant clearance angle of about 0 degrees. The segment II is connected to the flank corresponding to the second cutting edge. The segment II has a clearance angle that gradually increases from 0 degrees to the clearance angle of the flank connected to the second cutting edge. The flank connected to the second cutting edge has a constant dimension in the thickness direction and a constant clearance angle. The flank region A, which is connected to the first cutting edge, is connected to the flank connected to the second cutting edge.
However, the cutting insert described in Patent Document 1 is formed such that the transition region B, which is connected to the lower side of the flank region A connected to the first cutting edge, has the largest clearance angle among the flanks corresponding to the first cutting edge. That is, the flank region A, which is connected to the first cutting edge, protrudes by a large length toward a side of a body. Therefore, stress may concentrate on a connection portion between the transition region B and the flank region C connected to the lower side of the transition region B, and the stress may generate a crack in the connection portion. As a result, there is a problem in that the crack may cause a fracture of the cutting edge.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 7-276130